This invention relates to liquid metal cooled fast breeder nuclear reactor constructions of the pool kind.
A liquid metal cooled fast breeder nuclear reactor construction of the pool kind comprises a nuclear fuel assembly submerged in a pool of liquid metal coolant in a primary vessel which is suspended from the roof structure of a containment vault. Liquid metal cooled fast breeder nuclear reactors of the pool kind already in existence are of relatively small size, for example, the construction known as the Prototype Fast Reactor (PFR) situated at Dounreay in Scotland which generates 600 MW(t) of power. The fuel assembly in the PFR is carried from the roof structure by a series of ties but in some proposed new construction designs the fuel assemblies are carried from the wall of the primary vessel. Our co-pending U.S. Application Ser. No. 824235, now Pat. No. 4,172,010, by G. Seed, D. Hodgson and C. J. Grime for a 1300 MW(t) reactor, discloses a typical construction of this kind, the fuel assembly being carried on a strongback from the wall of the primary vessel by a skirt. The primary vessel and strongback, which would be manufactured in austenitic stainless steel, are subjected to deadweight loadings arising, in the case of the primary vessel, from the weight of the pool of coolant and the fuel assembly, and in the case of the strongback, arising from the weight of the fuel assembly. The primary vessel and strongback are also subject to pressure loadings and temperature gradients. In addition, weldments in these structures are also subjected to in-built stresses arising from the welding process. Because of the variations in fracture toughness properties of austenitic metal arising from temperature and irradiation effects, it has been postulated that small defects in weldments could give rise to fast crack propagation leading to catastrophic failure of components. The size and the rate of growth of such defects suggests that the integrity of austenitic stainless steel structures cannot be adequately assured by the more conventional means of detecting the onset of failure by periodic inspection. It is considered therefore that an alternative design philosophy should be adopted in which structures forming part of the main load line between the fuel assembly and the roof structure are at least tolerant to some degree of damage. This philosophy forms the basis of this invention.